  dict = Hash.new("")
  cont = true
  while (cont == true)
    puts "Nhap tu khoa: "
    k = gets.chomp.to_s
    
    check_available = (dict[k] ==  "") ? false : true
    if (check_available)
      puts(dict[k])
    else
      puts "Nhap dinh nghia:"
      val = gets.chomp.to_s
      dict[k] = val 
    end
    puts "Tiep tuc (yes or no):"
    ans = gets.chomp.to_s
    while (ans != "yes" && ans != "no")
      puts "Tiep tuc (yes or no):"
      ans = gets.chomp.to_s
    end
    if (ans == "no")
      cont = false
    end
  end